1. Field of the Invention
The subject invention generally pertains to a snap disc device, and more specifically pertains a snap disc device whose particular geometry provides exceptional operating characteristics.
2. Description of Related Art
The use of conventional compression springs can be limited by their physical size, as such springs are usually much longer than other types of springs. So, in applications where space is limited, other types of springs are often used, such as Bellville washers, curved disc springs, wave disc springs, and finger disc springs.
Belleville washers are resiliently compressible conical washers that provide a spring effect. For a given length, Belleville washers typically have much higher spring rates and significantly less travel than compression springs. This limits the use of Bellville washers to applications requiring relatively high forces and little travel. Belleville washers can be stacked back-to-back to provide lower spring rates and more travel, but a stack of washers will of course consume more space.
Curved disc springs have the shape of a flat washer that has been bent or bowed about a line parallel to the face of the washer. For a given size, a single curved disc springs may provide a lower spring rate than that of a Bellville washer. However, stacking curved disc springs to achieve even lower spring rates can be difficult to accomplish. Stacking the springs peak-to-peak is difficult to maintain, as the discs are normally free to rotate to a more stable arrangement of peak-to-valley.
Wave disc springs are similar to curved disc springs, but with more bends to create a wavy shape. Just as with curved disc springs, it can be difficult to maintain a stack of wave disc springs in a peak-to-peak arrangement. For a given size, wave disc springs tend to have less travel than curved disc springs.
Finger disc springs comprise an annular disc whose outer perimeter includes several fingers that are bent out of coplanar alignment with the rest of the disc. The fingers can resiliently deflect to create a spring-like effect. The fingers, however, may also interfere with being able to effectively stack finger disc springs with predictable results.
The physical structure of conventional disc springs limits their application. Current disc springs have limited use as springs and are not readily adapted for other uses such as gripping a square key.
Snap disc devices, invented by Pierre Schwab and disclosed in U.S. Pat. Nos. 4,822,959 and 5,269,499 have clover leaf shapes to create a bi-stable snap-action. However, the physical structure, operating characteristics, and/or method of pre-stressing such snap discs limits their usefulness.
To overcome the limitations of current disc springs and snap disc devices, an object of some embodiments of the invention is to provide an elastic disc that serves as a fastener by gripping the four sides of a square shaft.
Another object of some embodiments of the invention is to provide a fastener with a particular disc geometry that provides the fastener with a surprisingly high coefficient of compliance, and yet the fastener is readily stackable to lower or increase its spring rate.
Another object of some embodiments is to provide a fastener with a threaded member, wherein the fastener indicates the degree of tightness to which the threaded member compresses a bowed disc a predetermined amount of deflection against a standoff element.
Another object of some embodiments is to provide a disc-like fastener that helps inhibit a threaded fastener from unscrewing under vibration.
Another object of some embodiments is to provide a fastener with a threaded member, wherein the fastener indicates the degree of tightness to which the threaded member compresses a bowed disc a predetermined amount of deflection against a standoff element, and wherein the standoff element is a simple unitary ring.
Another object of some embodiments of the invention is to provide a fastener with a particular disc geometry that provides the fastener with a lower spring rate and more travel than a Belleville washer of similar material, thickness and diameter.
Another object of some embodiments of the invention is to provide a fastener with a particular disc geometry that provides the fastener with a higher coefficient of compliance than a Belleville washer of similar material, thickness and diameter.
Another object of some embodiments of the invention is to provide a fastener with a particular disc geometry that provides the fastener with spring characteristics that are generally between that of a Belleville washer and a compression spring.
Another object of the invention is to provide a cloverleaf shaped disc having a bowed shaped when it its normally unstressed position.
Another object of some embodiments is to provide a cloverleaf disc with radial protrusions around its outer perimeter that provide the disc with more freedom to deflect.
Another object is to provide a disc fastener that is radially symmetrical so it can be installed alone or in a stacked arrangement regardless of its rotational orientation.
Another object of some embodiments is to provide a disc fastener with ample travel and a significant spring rate even though the disk has a rather large diameter to thickness ratio.
Another object of some embodiments is to provide a disc fastener whose material thickness is less than 5% of its diameter, thereby making the disc especially useful where axial space is limited.
These and other objects of the invention are provided by a disc fastener having an outer edge and an inner edge, wherein portions of the inner edge extend radially further out than some portions of the outer edge. The disc""s geometry provides a coefficient of compliance greater than 10, wherein the coefficient of compliance is defined as the disc""s thickness cubed divided by a product of the disc""s spring constant at 75% compression times the disc""s effective outer diameter squared, wherein the thickness is expressed in mils, the spring constant is expressed in pounds per inch and the effective outer diameter is expressed in inches.